White dwarf feeding on red giant creates nova explosion

Astronomers using a pair of ground-based MAGIC telescopes have observed a massive nova explosion created by a pair of stars called RS Ophiuchi or RS Oph, located in the constellation of the Serpent.

The binary star pair consists of a small, extremely dense remnant of a formerly bright star called a white dwarf, along with a much larger red giant that is approaching the end of its life. The red giant sheds layers of hydrogen as its fuel dwindles, and this gas is sucked up by the dense white dwarf. But the sheer amount of gas engulfed by the white dwarf is overwhelming, and eventually the shell of gas that forms around it increases in temperature and pressure until it is ejected in a massive thermonuclear explosion. But that’s not the end of the story, because the two stars then continue the cycle.

Artwork of the binary star system RS Ophiuchi: Matter flows from the red giant to the white dwarf. The newly added stellar envelopes explode in a bright nova about every 15 years uperbossa/Max Planck Institute of Physics

It is one of these explosions, which occurs about once every 15 years, that the MAGIC telescopes have detected. The explosions are dramatic, giving off gamma rays with as much energy as 250 gigaelectronvolts, some of the highest energies ever seen in a nova.

The researchers were able to spot the explosions soon after being warned by other instruments to observe with the MAGIC telescopes. “The spectacular eruption of RS Ophiuchi shows that the rapid response of the MAGIC telescopes is really worth it: it takes them no more than 30 seconds to get to a new target,” said David Green of the Max Planck Institute for Physics, one of the authors, in a statement.

Observing the nova explosion was also valuable because it allowed the researchers to see its aftereffects as the explosion’s shockwaves propelled from the stars. This could help explain the origin of superfast particles racing through space, called cosmic rays.

“This also makes nova eruptions a source of cosmic rays,” Green said. “However, they tend to play the role of local heroes, meaning they only contribute to the cosmic rays in the vicinity. The big players for cosmic rays are the remnants of supernovae. The shock fronts created by stellar explosions are much more violent compared to novae.”

The research is published in the journal Nature Astronomy.

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